1. Field of the Invention
The present invention relates in general to a plastic optical element and a method and an apparatus for manufacturing the plastic optical element.
2. Discussion of the Background
In recent years, plastic-made optical elements are increasingly utilized because of the optical element's mass producability, lightness, and degree of freedom of shape or similar. Such increased usage of optical plastics is because of the optical plastics' characteristics of low sponginess and low double refractivity. A molding method for the optical plastics, such as an injection compression molding method, satisfies both the shape precision and low double refraction characteristic.
Besides the shape precision and the low double refraction characteristic, it is currently required that the distribution of a refractive index in an optical element be small, while also requiring a more highly precise plastic optical element. The refractive index generally varies the larger the optical element is and the closer the optical element is to a surface of a molding item (i.e., lens), in contrast to the smaller the optical element is and the closer it is to a center of the molding item. Such a distribution can generally be a cause of deviation of the focusing position in a focusing lens.
For example, as a light scanning lens, Japanese Patent Application Laid Open No. 10-288749 (hereinafter JP '749) describes the refractive index varying in such a manner that a light spot, which is to condense on a scanning operation receiving surface, tends to distance itself from a light deflection instrument and passes through a predetermined designed position. As a result, a diameter of the light spot on the scanning operation receiving surface becomes larger than the designed value, and accordingly, image quality of a recording image written by such a light scanning operation is lowered.
Further, it is currently believed that the distribution of the refractive index is produced by the following. Namely, a molding item is cooled too fast when its temperature is higher than a glass transition point or a heat deformation temperature in a plastic cooling and consolidating process of a molding operation, and accordingly, plastic density becomes uneven.
To avoid the influence of the refractive index distribution, the following technologies are heretofore proposed. For example, Japanese Patent Application No. 8-201717 (hereinafter JP '717) describes that a lens shape is defined by the inequality, as follows:H/T>2,wherein T represents thickness in a beam advancing direction of the lens, and H represents height perpendicular to the beam advancing direction.
According to the technology, the refractive index distribution in a beam transmission region is decreased, because the height is increased, and accordingly, the temperature distribution in the beam transmission region during a plastic cooling process becomes relatively smaller.
Further, Japanese Patent Application No. 9-49976 (hereinafter JP '976) proposes a technology wherein a beam is focused on a scanning operation receiving surface by increasing the refraction force of a scanning operation receiving the surface side of the focusing lens so that the focusing positional deviation caused by the refractive index distribution of a focusing lens is corrected.
Further, Japanese Patent Application No. 9-109165 (hereinafter JP '165) proposes a technology wherein the refractive index distribution is decreased by applying the process, as follows:
Namely, an injection compression-molding item is subjected to an annealing process for more than two hours under a temperature ranging from −55° C. to −25° C. of a glass transition point of the optical plastic material as a reference.
JP '717 has the disadvantage that the cost of production is greatly increased, because both the amount of plastic to be used is increased and the cooling time period required is increased.
JP '976 has the disadvantage that a correcting value needs to be determined and evaluated, after a lens is molded and processed under fixed molding conditions. The correcting value may need to be changed and the shape should be corrected again, if the molding conditions are changed. Additionally, the correction should be made per cavity, whereby the development of a mold could be enormously costly, if a mold capable of producing a plurality of items at once is utilized. Further, all molded products become invalid, and accordingly, a yielding rate is lowered, even if the refractive index distribution varies only slightly because of the unstableness of the molding conditions.
JP '165 has the disadvantages that although it is possible decrease the refraction index distribution up to a prescribed level, such a decreased refraction index distribution is insufficient for a highly precise lens. In addition, the time period for the annealing process may be long (i.e., up to a few hours) and become extremely expensive.
Thus, it would be desirable to develop a technology for widely decreasing the refractive index distribution, while obtaining a low cost and highly precise optical element.